Among so many systems already known able to detect an intrusion, the most reliable are the ones of optoelectronic kind. They consist of a projector which directs a beam of infrared rays into a facing receiver. When the intruder, passing between the projector and the receiver, intercepts the beam, the very fact that the excitation of the receiver is broken gives rise to the information that an intrusion is occuring.
The drawback of these systems is that they need immovable installations. In fact the projector and the receiver must be mounted into a fixed and separated support and must be well aligned. When the distance between the units exceed a few meters, the alignment becomes difficult also because the infrared radiation projected is invisible. Therefore not only the device cannot be easily displaced everytime at will of the user, but, considering also the aesthetic requirements of the room, it is to install only in particular places as for instance the door or window spaces. The limitations to the choice of the vigilance direction, in practice, involves the need of several apparatus in order to achieve a sufficient degree of defence.